What side effects are expected with the vaccines?

It is important to note that the vaccine cannot give anyone the virus. Sore arm, injection site soreness and/or redness, some experience systemic side effects/immune response like muscle aches, fatigue, low grade fever, some reported a headache. These symptoms are likely only to last 1-2 days, no more than one week and much less severe than actually getting the virus itself.


In the Pfizer-BioNTech clinical trial, 77.4% reported at least one systemic reaction during the seven days after vaccination. The frequency of systemic adverse events was higher in the younger age group than the older age group (82.8% vs 70.6%). Within each age group, the frequency and severity of systemic adverse events was higher after dose two than dose one (with the exception of vomiting and diarrhea which were similar between doses and between vaccine and placebo).


In the Moderna clinical trial, systemic reactions were reported by the majority of vaccine recipients and at higher rates than placebo recipients. The frequency of systemic reactions among vaccine recipients was higher in the younger age group (18-64 yo) than the older age group (81.9% vs 71.9% after dose 2). Within each age group, the frequency and severity of systemic reactions was higher after dose 2 than dose 1. For both age groups, fatigue, headache and myalgia were the most common.

Johnson & Johnson

The most common adverse reactions reported in the clinical trial following administration of the vaccine included injection site pain (48.6%), headache (38.9%), fatigue (38.2%), myalgia (33.2%), and nausea (14.2%). Severe allergic reactions, including one case of anaphylaxis in an ongoing open-label study in South Africa, have been reported following the Janssen COVID-19 vaccine administered in clinical studies. The frequency of systemic reactions was higher in participants aged 18-59 years than participants ≥60 years (61.5% vs 45.3%). For both age groups, fatigue and headache were the most commonly reported systemic reactions. Fever was more common in participants 18-59 years (12.8%) compared to those ≥60 years (3.1%). Among vaccine recipients, the median onset of systemic reactions within 2 days of vaccination, with a median duration of 1-2 days.

On April 25, 2021, after a brief pause, CDC and FDA recommended use of Johnson & Johnson’s Janssen COVID-19 vaccine resume in the United States. Previously, administration of this vaccine was paused on April 12 after reports of a rare and severe type of blood clot in 6 U.S. individuals and 1 death.

After a review of all available data, CDC and FDA have concluded that at this time, J&J/Janssen COVID-19 vaccine’s known and potential benefits outweigh its known and potential risks. CDC also states on its website that women younger than 50 years old especially should be aware of the rare but increased risk of this adverse event and that there are other COVID-19 vaccine options available for which this risk has not been seen.

Not everyone will experience significant side effects from these vaccines. This does NOT mean the vaccine isn’t working. The CDC is actively collecting info about side effects – sign up with V-safe after getting the vaccine and get texts that ask several questions about how you feel. This is part of the ongoing active monitoring for safety.


Updated: May 5, 2021